Temperature sensors are used in automation technology in the most varied of applications. A large number of different temperature sensors are produced and sold by the applicant. Often, temperature sensors are secured on a container wall (tube, or pipe, wall, tank wall) in such a manner that the temperature sensitive component of the temperature sensor is in thermal contact with a process-located medium, whose temperature is to be determined or monitored.
A basic problem for a stable and reliable temperature measurement is drift of the temperature sensors. This drift means that the temperature sensors have to be calibrated in certain time intervals. In such case, the time intervals between the calibrations are strongly dependent on the particular embodiment of the temperature sensor and the conditions, which reign at the location of use of the temperature sensor in the process.
For purposes of the calibration, a temperature sensor must usually be deinstalled and brought in contact with a suitable calibration medium of defined temperature, e.g. ice water. If the temperature value of the temperature sensor measured in the calibration medium deviates from the known reference temperature of the calibration medium, then the measured temperature value is corrected to the reference temperature. The deinstallation of the temperature sensor is problematic, since it often makes an interruption of the process necessary—and a shutdown of a plant is usually connected with costs for the plant operator. Additionally, in the case of the deinstallation, a sealed connection between temperature sensor and container wall is opened and must subsequently be re-created.
Known from patent EP 1247268 B2 is an in situ calibrating with a number of integrated temperature sensors. Installed in a measuring insert is, supplementally to a primary temperature sensor, one or more reference temperature sensors. A reference temperature sensor differs as regards construction and utilized materials from the primary temperature sensor. Apart from the fact that, in the case of the known solution, a number of temperature sensors must be applied, in the case of a reference temperature sensor, aging effects and characteristic line drifts can occur, which deviate from those of the primary temperature sensor. Thus, according to an embodiment, used in parallel with a primary Pt100 resistance sensor as reference temperature sensors are semiconductor temperature sensors—so-called NTC/PTC resistors. In order that the temperature measurement work reliably in the case of the known solution, the characteristic lines and the aging characteristics of the reference temperature sensors must be known very exactly; at least, however, the characteristic line changes of the reference temperature sensors must be lower than in the case of the primary temperature sensor to be monitored.
Known from DE 10 2010 040 039 A1 is a temperature sensor with a reference element. The reference element is composed partially of a ferroelectric material. Ferroelectric materials experience a phase transformation at a certain, defined temperature value (Curie temperature). As a result of the phase transformation, the electrical conductivity changes abruptly. If the known temperature sensor is working in a temperature range, in which the phase transformation occurs, then this can be detected and the measured temperature value, in the case of a deviation, is calibrated, i.e. corrected, to the certain, defined temperature. From the preceding description, it can be seen that the known in situ calibration solutions only function with specially manufactured temperature sensors.